Aluminum alloy



3,020,154 Patented Feb. 6, 1962 ALLOY Noble N. Ida, Towson, Md.,assignor to Martin-Marietta Corporation, a corporation of Maryland NoDrawing. Filed Apr. 24, 1953, Ser. No. 730,535 3 Claims. (Cl. 75-138)This invention relates to aluminum-base alloys especially suited for usein pressurized-water power reactors, and a method of preparing same.

Because extreme thermal and nuclear conditions obtain in the centralregion of a reactor, the selection of materials from which cores may befabricated is limited. In research and critical reactors, which operateat low temperatures, core elements are generally constructed of aluminumin preference to stainless steel or zirconium. The application ofaluminum metals as fuel element cladding reduces reprocessing costsappreciably below that for stainless steel or zirconium elements.Aluminum and aluminum-base alloys recommend themselves also because oftheir low thermal neutron absorption cross sections, relative stabilitytowards radiation effects and excellent fabn'cabi-lity characteristics.

In view of these outstanding properties it would be highly desirable toutilize aluminum in pressurized-water power reactor cores. However, atthe temperatures usually attained in such reactors, from about 500 F.upwards, almost all commercial aluminum alloys are severely corroded bywater. Consequently, stainless steel and an. alloy of zirconium,Zircaloy 2, are the materials of choice. Both are considerably moreexpensive than alu minum. For example, an estimated saving of 80% inmaterials cost would be realized by cladding fuel elements with aluminumrather than zirconium. In comparison with stainless steel, aluminum hasa much smaller absorption cross section for thermal neutrons, so thatfor a given power output less fissionable material is required inaluminum reactor cores than in stainless steel cores of similar design.

It is the purpose of this invention to provide aluminumbase alloyshaving excellent water-corrosion resistance and strength up to about 700F. These alloys are especially useful in pressurized-water reactors ascladding for fuel elements, and are characterized by small thermalneutron absorption cross sections, excellent fabricability and lowmaterials and reprocessing costs. It is expected that these alloys willbe relatively stable towards radiation effects.

In accordance with the present invention an aluminum alloy havingsuperior resistance to water corrosion up to about 700 F. as comparedwith existing commercial aluminum alloys is prepared from aluminum,nickel and titanium with or without additions of silicon and niobium.

The ranges by weight percent of the ingredients of the alloys are asfollows:

Nickel 0.5 5.0

Titanium 0.5 3.5 Silicon 0.0- 1.0 Niobium 0.0 2.5 Aluminum Balance Thegeneral practise, heretofore, was to limit the percentages of titaniumand nickel in aluminum to approximately 0.3% and 1.5%, respectively,since higher concentrations tend to cause segregation in andembrittlement of the resulting alloy. Accordingly, persons skilled inthe art will readily appreciate the fact that unusually large amounts ofnickel and titanium are used in the practise of this invention. Contraryto a priori predictions it has been determined that embrittlement andsegregation can be largely inhibited it special measures are taken toinsure uniform distribution of the precipitating phases and to suppressthe formation of large grains during castmg.

A preferred melting practise followed in the preparation of these alloysemploys standard apparatus. Alloying and pouring may be performed inair. After homogenation of the melt, it is cleared of dross, cast,partially cooled and rapid-quenched in water. In this manner graingrowth is minimized and precipitation of intermetallics occurs uniformlythroughout the cast ingot.

Although the accumulation of oxide dross during air melting of thesealloys is not excessive, it can be reduced by the usual methods. Forexample, just prior to pouring, chlorine may be bubbled through themelt. Again, an empirically determined amount of AlCl or CaF about 2% to10% by weight of the over-all melt, may be added.

A particular melting program found satisfactory will now be described.Predetermined quantities of aluminum, nickel and titanium, together withany additions of silicon or niobium or both, are melted in a graphite orzirconia crucible in a high-frequency induction-heated air meltingfurnace, and brought to a temperature of about 1450 F. This temperatureis held for 2 to 5 hours so that melting and homogenation are complete.At the end of this period, dross is removed manually and the moltenalloy is poured into a stainless steel mold. The mold may be paintedwith zirconia or other suitable refractory material to eliminatesticking of the ingot to the mold. The ingot, at a temperature of from900 to 1100 F., is quenched in water. Grain growth and phase segregationare kept to a minimum by quenching at the higher temperatures. It hasbeen observed, however, that neither phenomena present a problem if theingot is not allowed to cool below about 900 F. before quenching. Thecast structure is then homogenized at a temperature between about 500 C.and 600 C. for a minimum of six hours.

Microscopic and analytical examinations of homogenized ingots preparedby the above method have established that precipitation of the variousphases is uniform throughout, with grain size averaging about ASTM No.4. In the ternary alloys, i.e., Al-Ni-Ti alloys, TiAl and NiAlstructures have been identified.

ingots prepared as above are readily rolled into strip of. any desiredthickness provided precautions are taken to prevent stringering andconsequent embrittlement, which would cause edge cracking. I have foundthat hot rolling at about 450 C. gives excellent results. Prior toworking, the ingot is preannealed for one hour at about 500-60 0 C. Theannealed ingot is then given several reduction passes with intermediatehalf-hour anneals at 450 C. so that a working temperature ofapproximately 450 C. is maintained. Depending upon the size of therollers, reductions of up to 60% per pass are possible. It is preferredto hot roll until a thickness of 0.250 inch is reached, and solutionheat-treat the rolled strip at 620 F. for 2 hours. This soaking step isfollowed by a rapid quench in water. Further reduction can then beaccomplished by cold rolling. In order to remove the effects of coldworking, the final strip may be annealed at 800 F. for about one-halfhour.

The compositions by Weight percent of several representative melts areas follows:

A series of corrosion tests on alloys of this invention and comparativestudies of two aluminum alloys presently marketed were carried onsimultaneously. I The two it is apparent that the subject alloys willhave longer lifetimes in aqueous environments than the best aluminumalloys presently available for such use. In pressurizedwater reactors,for example, 10 mil fuel element claddings of either G or F can beexpected to be serviceable for a period of at least one year. Claddingwith 10 mils of alloy B extends this period considerably, such that,prescinding from a consideration of radiation effects, the cladding ofalloy B will have a lifetime twenty times greater thanrthat of eithercommercial alloy.

The present alloys also exhibit a remarkable improvement in mechanicalproperties as compared to F and G.

alloys are designated herein as F and G, were specifically Temp Ultimatedeveloped for high temperature water systems. Their Alloy g sgcompositions are as follows:

200 25, 500 Alloy Composition by Weight Percent 500 11,700 600 6,385 0009,000 F 6 1 Cu, 0.3 M11, 0.1 v, 0.15 Zr, balance A1. 600 7, 220 G 1 0Ni, 0.5 Fe, 0.2 Si, balance 1. (S00 8, 385

Corrosion test specimens cut from a rolled sheet were prepared byconventional cleaning methods. The specimens were placed in an autoclavemaintained at con stant pressure and temperature, and, at the end ofeach run, examined for surface appearance, flaking, penetration andweight loss. In no instance did the subject alloys exhibit any sign of awhite corrosion product; rather a very thin and tightly adherentprotective oxide layer having a dark or jet black appearance was formed.In contrast, white flaky corrosion products were present on the twocommercial alloys. Other corrosion data are presented in Table I.

. TABLE I Typical water corrosion rates of the present alloys comparedwith F and G On the basis of the corrosion results listed in Table IIncreases in high temperature tensile strength of 30% or more have beenrealized. Several specimens, fully annealed, were pulled at varioustemperatures, and the results are listed above.

I claim:

1. An aluminum alloy consisting by weight of about 0.5% to 2.0% nickel,1.0% to 3.5% titanium and the balance being essentially aluminum.

2. An aluminum alloy consisting by weight of about 0.5% to 2.0% nickel,about 1.0% to 3.5% titanium, not more than about 0.5% silicon and thebalance being essentially aluminum.

3. An aluminum alloy consisting by weight of about 0.5% to 2.0% nickel,about 1.0% to 3.5% titanium, silicon and niobium, each in amounts notmore than about 0.5%, and the balance being essentially aluminum.

References Cited in the file of this patent V UNITED STATES PATENTS2,263,823 Bonsack et al Nov. 25, 1941 2,463,022 Cooper et al Mar. 1,1949 2,578,098 Southard Dec. 11, 1951 2,579,369 Dawe Dec. 18, 19512,781,261 Kamlet Feb. 12, 1957 2,871,176 Draley et al. Jan. 27, 1959

1. AN ALUMINUM ALLOY CONSISTING BY WEIGHT OF ABOUT 0.5% TO 2.0% NICKEL,1.0% TO 3.5% TITANIUM AND THE BALANCE BEING ESSENTIALLY ALUMINUM.